Metals are phosphated to form metal phosphate layers which are firmly intergrown with the metal surface and which in themselves improve the resistance to corrosion and in combination with paints and other organic coating will contribute to a substantial increase of the adhesion and of the resistance to subsurface corrosion. Phosphate layers also serve as insulation against passage of electric currents and, in combination with lubricants, will reduce sliding friction.
A pretreatment before a painting operation may particularly be effected by a low-zinc phosphating process, in which phosphating solutions are employed which have relatively low contents of zinc ions amounting to 0.5 to 1.5 g/l. The phosphate layers formed on steel under such conditions will have a high content of phosphophyllite (Zn.sub.2 Fe(PO.sub.4).sub.2.4H.sub.2 O), which has a much higher resistance to corrosion than the hopeite (Zn.sub.3 (PO.sub.4).sub.2.4H.sub.2 O) deposited from relatively high-zinc phosphating solutions. If the low-zinc phosphating solutions contain also nickel ions and/or manganese ions, the protection afforded in the presence of paints will be increased further.
Low-zinc processes in which 0.5 to 1.5 g/l manganese ions and 0.3 to 2.0 g/l nickel ions are employed are widely used as so-called three-cation processes to prepare metal surfaces for a painting operation, e.g. before a cathodic electro-dip lacquer coating of automobile car bodies.
These processes are disadvantageous due to the high content of nickel ions in the phosphating solutions used in the three-cation processes and to the high content of nickel and of nickel compounds in the resulting phosphate layers, because nickel and nickel compounds are undesirable from the aspects of workplace hygiene and pollution of the environment.